The spine is the axis of the skeleton on which all of the body parts hang. In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar spine sits upon the sacrum, which then attaches to the pelvis, and in turn is supported by the hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints but allow known degrees of flexion, extension, lateral bending, and axial rotation and translation.
Typical vertebra has a thick anterior bone mass called the vertebral body, with a neural (vertebral) arch that arises from the posterior surface of the vertebral body. The central of adjacent vertebrae are supported by intervertebral discs. The spinal disc and/or vertebral bodies may be displaced or damaged due to trauma, disease, degenerative defects, or wear over an extended period of time. One result of this displacement or damage to a spinal disc or vertebral body may be chronic back pain. In many cases, to alleviate back pain from degenerated of herniated discs, the disc is removed along with all or part of at least one neighboring vertebrae and is replaced by an implant that promotes fusion of the remaining bony anatomy.
The success or failure of spinal fusion may depend upon several factors. For instance the spacer or implant or cage used to fill the space left by the removed disc and bony anatomy must be sufficiently strong to support the spine under a wide range of loading conditions. The spacer should also be configured so that it likely to remain in place once it has been positioned in the spine by the surgeon. Additionally the material used for the spacer should be biocompatible material and should have a configured that promotes bony ingrowth.
Cages have been developed to help support the spine and maintain the normal spacing between opposing vertebrae. Typically, cages are pre-manufactured at various heights requiring that a cavity between opposing vertebrae be prepared and distracted to a dimension corresponding to the most suitably sized cage. The surgical procedure to prepare the implant site can be difficult and lengthy. Moreover, the procedure can increase risk of trauma to the tissues surrounding of the implant site.
Distractible cages may be used as both a fusion device and/or a means for maintaining intervertebral spacing. Often these implants include a drive means that allows the cage to be expanded in situ to a size that corresponds to the cavity created when the damaged tissue is removed. The drive means typically includes devices such as gears, threaded rods, and the like, in mechanical engagement so as to expand or contract the device to a necessary distance between the vertebrae.
There are a number of known conventional fusion devices and methodologies in the art for accomplishing the intervertebral fusion. These include screw and rod arrangements, solid bone implants, and fusion devices which include a cage or other implant mechanism which, typically, is packed with bone and/or bone growth inducing substances (i.e., bone graft). These devices are implanted between adjacent vertebral bodies in order to fuse the vertebral bodies together, alleviating the associated pain. Examples of such fusion devices are taught and described the following U.S. Pat. Nos. 6,830,589, 8,062,375, 8,435,298, 8,518,087, and 8,556,979 and the
There are drawbacks associated with the known conventional fusion devices and methodologies. For example, while current expandable cages are designed to expand in height, their lateral profile remains the same (or is decreased) as they are expanded. Accordingly, there is a need for expandable fusion devices that expand laterally and a further need for expandable fusion devices that can expand laterally.
Moreover, it is advantageous to pack cages with bone graft. In view of the gears, threaded rods, and the like various used to expand the cages, packing such bone graft is more difficult. Accordingly, there is a need for an improved expandable fusion device that has the open volume for the bone graft and openings (i.e., graft windows) that render this open volume readily accessible.